VS2015 warnings C*

[lukaszstolarczuk]

multiple warnings (currently 216) are produced in the build on VS 2015

e.g.:
C:\Program Files (x86)\Windows Kits\8.1\Include\um\DbgHelp.h(1544): warning C4091: 'typedef ': ignored on left of '' when no variable is declared [D:\qba1\workspace\24912\libpmemobj-cpp\build\tests\test_backtrace.vcxproj]
or
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\include\xmemory0(586): warning C4494: 'std::allocator_traits<pmem::obj::allocator<pmem::detail::pair<const Key,Value>,pmem::obj::standard_alloc_policy<T>,pmem::obj::object_traits<T>>>::allocate' : Ignoring __declspec(allocator) because the function return type is not a pointer or reference [D:\qba1\workspace\24912\libpmemobj-cpp\build\tests\concurrent_map_tx.vcxproj]